Hydra HDRI Pro - Make a 32-bit HDRI From an 8-bit Image

Hydra Design Labs' Hydra HDRI Pro plugin claims to convert an
ordinary 8-bit image such as a JPG into a usable 32-bit per pixel
image, suitable for High Dynamic Range Image based rendering. Snap
a panoramic with your phone and use it as as an HDRI light source
in your renders. In their own words:

"Hydra HDRI Pro is a photoshop plugin
that will convert an 8-bit image into a 32-bit HDRI. Create your
own spherical HDRI's for use in 3D rendering programs. Take your
8-bit spherical image, edit it in Photoshop, and then with a click
of a button convert it into an HDRI."

I was skeptical. I had to have a closer look! Hydra Design Labs
sent me a copy of Hydra HDRI Pro Photoshop plugin to evaluate.
Hydra HDRI Pro retails for around $150. This review is my
thoughts after testing it on Windows 7 using Photoshop CS6 and
using Autodesk Maya's Mentalray for the rendering aspects, but you
could use any modern renderer.

This image was rendered with an HDRI light source created from an
8-bit per pixel image. Image courtesy Hydra Design Labs.

A little background: (Advanced readers can skip ahead.) A High
Dynamic Range Image (HDRI) contains wider range of exposures than
ordinary JPGs. A JPG can hold 0-255 possible luminance values, i.e.
256 in total. (That's 2^8, because a JPG is 8-bits per color
component.) If we adjust the luminance of a JPG image, we simply
can't recover areas of black shadow or blown-out highlights, the
data simply isn't there.

If we took an exposure using "RAW" mode we would get
roughly 14-bits per channel of luminance data per pixel. We then
have to convert these "RAW"s to TIFFs or openEXR or some
other format our renderer can read. Even so, that's only 16,384
luminance values. We can do so much better.

If we instead took a series of 14-bits per channel images and
spaced the exposure times out to cover a wider range, we could have
a potentially unlimited dynamic range. Typically we use a 32-bit
per pixel image format such as TIFF, HDR, or openEXR. This gives us
potentially 4,294,967,296 luminance values.

Who Cares?

Contrast cares! By the time light emitted from an HDRI bounces
around the scene, gets reflected and refracted a few times, there's
potential for a very wide range of luminance values. If we use only
an 8-bit per pixel image as a light source we often end up with
poor contrast.

Shouldn't we just use a proper HDRI in the fist place?

If possible, yes. However not everyone owns a DSLR camera and
the software to make a 'true' HDRI. The idea behind Hydra HDRI
Pro is you can use a ordinary camera like your phone to
capture 8-bit images and then convert them to 32-bit per channel
HDRIs that are "good enough" for rendering. It's a
convenience vs. quality tradeoff.

An added benefit is you can use ordinary image editing tools on
the 8-bit version and only convert it to a 32 bit HDRI after all
edits are done. Even Photoshop as of CS6 is limited in its ability
to manipulate 32-bit images. Therefore this technique opens the
possibility to use all available tools.

Image courtesy Hydra Design Labs.

How is this different from Image -> Mode -> 32-Bits?

The output file is re-normalized to a 32-bit space, not simply
converted! You could accomplish the same feat by duplicating the
image several times, adjusting the luminance values of each layer
to approximate a traditional HDRI and re-stack them. In fact,
that's what the plugin appears to do, it's just it does it much
faster than doing so by hand. There's probably some other magic in
there too.

Does it actually work?

Yes, to the extent possible it works rather well! Understand
that no computation can ever recover luminance information that
wasn't there in the first place. Hydra HDRI Pro makes no
claim to do so. What it does accomplish however, is better contrast
in the resulting renders than using the virgin 8-bit versions
images as light sources. If all you have in your arsenal, or on
your person, is a smart phone you're usually stuck with 8-bit per
pixel images. These situations are where Hydra HDRI Pro
can help you turn those 8-bit images into suitable 32-bit
HDRIs.

Image courtesy Hydra Design Labs.

Hydra HDRI Pro is cake to use. There's no settings and
only a single button. Once installed, you simply open an ordinary
8-bit JPG, such as a panoramic you captured (The plugin itself does
not stitch panoramics. That's another subject entirely.). You then
select File -> Scripts -> Hydra HDRI Pro, click
Convert and let the script work.

Image courtesy Hydra Design Labs.

A few moments later it spits out a 32-bit version of the same
image, saved as Untitled_HDR2.hdr, in the same directory as the
original image. We then apply this new 32-bit image as a light
source in a renderer that supports HDRI based lighting.

Image courtesy Hydra Design Labs.

Show Me The Goods

Below is a comparison render. The image on the left uses an
ordinary JPG as an HDR light source, both rendered with Autodesk
Mentalray. Notice how the contrast is lower and the highlights a
bit blown out on the left car.

Left is ordinary JPG used as a light source. Right is same JPG
converted into an HDRI used as a light source.

The image on the right used the same exact JPG source image, but
it was first converted into an HDRI file by Hydra HDRI
Pro. Note the improvement in contrast, especially around the
headlights. This is a subtle example but the difference is still
readily apparent. The effect becomes even more dramatic when you
start to boost the Color Gain to brighten or darken the image used
as the light source.

Bottom Line

It's a quality vs. convenience tradeoff. Due to the ubiquity of
cameras that capture at 8-bits per pixel, such as most phone
cameras, Hydra HDRI Pro opens up the possibility of taking
an impromptu panoramic and then using it as an HDRI light source.
There's also the ubiquity of tools to manipulate 8-bit per pixel
images. You can use any tool you want to tweak your images prior to
HDRI conversion.

Hydra HDRI Pro also means that mere mortals can create
their own HDRI library using inexpensive tools. These are the main
selling points and rightfully so. Hydra HDRI Pro does
indeed do what it claims. The rendered results in most cases are
acceptable for production use such as product shots, etc.

Cons: Hydra HDRI Pro can never match the quality of an
image taken at higher bit depth in the first place. If you have all
the expensive cameras and other tools to make "proper"
HDR images, Hydra HDRI Pro can't beat them for quality.
With Hydra HDRI Pro you also don't have as much
flexibility of tweaking the luminance of the HDR image in your
render engine. You can do it, but you don't retain the full
spectrum of color that you would have with a higher bit depth
source image because the data simply wasn't in the 8-bit image to
begin with.

The plugin itself has some minor thorns. The most notable is
that you can't name the output image. It always gets named
Untitled_HDR2.hdr, even if you have an image in the source folder
that is already using that name - It will be silently overwritten.
I would also love to see a stand-alone command line version of the
program that can conveniently operate on batches of files in the
background.

Again, quality vs. convenience. Hydra HDRI Pro is
indeed convenient. Are the resulting HDR images high quality enough
for your needs? Only you can decide that (I'd say 90% of the time,
yes.). Unfortunately, there isn't a demo of Hydra HDRI Pro
at this time, but I bothered Jon Hull, President of Hydra Design
Labs enough that we just might see one in the near future!

System Requirements

Hydra HDRI Pro is compatible with the following
versions of Adobe Photoshop:

Kurt Foster (Modulok) falls somewhere between
programmer and visual effects artist. When not sifting through
technical manuals, he takes on freelance roles in both programming
and visual effects, attempting to create a marriage of technical
knowledge with artistic talent. He can be seen helping out on the
Renderosity Maya forum, when time permits.

May 12, 2014

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